In a power-off electromagnetic brake used in servomotors and the like, an armature is urged toward the rotating shaft of a motor by a spring force and brake force operates in an unexcited state. When excited, the armature is attracted toward the yoke by magnetic attraction force, and the brake force operating on the rotating shaft of the motor is released. A considerable amount of power is required when the armature is attracted to the yoke, but the power required to hold the armature in an attracted state after the armature has been attracted to the yoke can be a small amount. In other words, the power for maintaining the attracted/held state of the armature that is used for holding a brake-free state can be a small amount.
In Patent Document 1, two sets of exciting coils of a power-off electromagnetic brake are connected in series. One of the sets of exciting coils is used as a low-resistance coil for momentary attraction through which a large electric current flows for a short interval of time at the start of brake release, and the other set of exciting coils is used as a high-resistance holding coil in which a small electric current flows for holding the armature over a long period of time after attraction, whereby unnecessary electric current is prevented from flowing when the brake is in a released state. In Patent Document 2, a rectifier circuit, which is used for obtaining a DC power source from an AC power source in an electromagnetic brake used in an AC induction motor, is used to briefly send a full-wave rectified direct current to the coil winding of an electromagnetic brake when the armature is attracted. The rectifier circuit is configured to send half-wave rectified direct current after armature attraction, and the power that is supplied after the armature of the electromagnetic brake has been attracted is made to be less than the power that is supplied during attraction.    Patent Document 1 JP-A 2000-304073    Patent Document 2 JP-A 2001-349358
With the electromagnetic brake disclosed in Patent Document 1, the structure cannot be reduced in size because two exciting coils are provided and the cost is high. With the electromagnetic brake disclosed in Patent Document 2, the use of an AC power source is presumed, and the brake does not adequately function with direct current that has been subjected to half-wave rectification when the torque startup time of the brake is shorter than ½ the power source frequency.